Publication date: 15th May 2025
We use Raman spectroscopy to probe the vibrational and structural properties of alloyed and heterostructured nanocrystals and nanoplatelets based on CdSe and CdS. The alloyed spherical quantum dot series CdSe1-xSx includes CdSe, CdSe0.75S0.25, CdSe0.50S0.50, CdSe0.25S0.75, and CdS. The longitudinal optical (LO) phonon frequency for both the Cd-Se and Cd-S stretches decreases as the selenium and sulfur content of the alloys decreases, respectively. These trends with composition are consistent with "two-mode" behavior that is characteristic of homogeneous alloys of CdSe/CdS. In temperature-dependent measurements, the CdSe LO peak frequency increases with decreasing temperature, while the CdS LO peak frequency remains unchanged, suggesting stronger phonon-phonon coupling in the Cd-Se vibrations. Alloyed and heterostructured nanoplatelets that are four monolayers thick exhibit similar trends. In a nanoplatelet alloy series that ranges from pure CdSe to pure CdS, we again observe two-mode behavior that indicates strong intermixing of selenium and sulfur. Both CdS/CdSe core/shell quantum dots and CdS/CdSe core/crown nanoplatelets have Raman spectra that are distinct from the corresponding homogeneous alloy of comparable overall composition, likely due to different degrees of strain and phonon confinement in the heterostructures. Interestingly, both the Cd-Se and Cd-S LO mode frequencies change very little between fully homogeneous nanoplatelet alloys and CdS/CdSe core/crown nanoplatelets with a graded interface, indicating that the LO mode is not sufficient to evaluate the length of the interfacial region.
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